This background concerns the froth flotation of oxidized coal and coal-containing solids. In particular, the process involves the use of a novel conditioner class: fatty esters of alkanolamine hydroxyalkylates.
Coal occurs naturally in several different forms, according to local geology. However, the coal happens to occur, one should expect to find varying amounts of undesirable materials present with the carbonaceous solids. For example, sulfur content may be objectionably high in raw coal. Likewise, ash and generally noncombustible materials (such as sand) may be found along with the coal. Whatever the unwanted substances, solid ores in general and coal in particular have been the subject of study relating to the separation of the desired product from the undesired balance.
Froth flotation is well known as an effective method of carrying out such separation. This process exploits a physical difference between the wanted and unwanted solids, to wit, one type of solid particle preferentially wets while the other does not. In that way, agitation of a slurry creates a bubbly surface region in which one type of the pulverized solids may be found. For example, consider a solid mass of materials X and Y. Froth flotation separation of X from Y first requires the solid mass to be pulverized into small particles of different compositions--some particles with more X and some with less X. Next, the particles are agitated in a carefully chosen liquid (probably a mixture rather than a pure liquid). The wetted particles, say predominantly X, remain in the bulk liquid while the unwetted particles, say predominantly Y, are located in the air bubbles of the froth. A simple embodiment of the general idea appears in U.S. Pat. No. 2,389,763. That reference involves removal of silica and siliceous materials from manganese and magnesium ores. The flotation agent in said reference is the heart of the invention, consisting in this case of aliphatic carboxylic acid esters of nontertiary alkylamines. Experience has taught that the precise structure of the flotation agent is critical to the effectiveness of a given separation. This result comes as no surprise, since liquid-liquid extraction and chromatography also exhibit critical sensitivity to small changes in chemical structure of the mixture components.
The specific context of the present invention involves oxidized coal. Such coal has generally been known to be somewhat separable from solid ash, using an air-water system for froth flotation where the water contains some carefully chosen flotation agent. The liquid may contain additional components, such as chemicals intended to adjust the pH or induce bubble formation. In any case successful separation of coal from ash has depended on making the coal particles surface sufficiently hydrophobic. Alkanolamines and their derivatives function reasonably well in industrial separation processes, but since froth flotation of coal depends critically on the exact makeup of the liquid, there remains the question of exactly what flotation agent is appropriate.
U.S. Pat. No. 4,474,619 teaches the use of a condensate of an alkanolamine with at least 0.8 equivalent of a fatty acid. Condensation reactions such as esterification are generally reversible, so the composition of the products will depend strongly on the composition and stoichiometry of the reactants. This reference notes that the preferred ratio of fatty acid or fatty ester to alkanolamine is about 2:1 or 3:1 for yield of the most effective conditioner. A refined kerosene (Soltrol 100) serves as the collector, and the slurry also contained a frother (Dowfroth 1012). Furthermore, this reference recognizes the efficacy of diethanolamine condensed with a commercially available mixture of rosin acids, oleic acid, linoleic acid, stearic acid, etc. This mixture of tall oil fatty acids is generally denominated TOFA, while diethanolamine is denoted by DEA. Thus, one may speak of a 1:2 condensation of DEA/TOFA, for example. A comparison of the present invention with U.S. Pat. No. 4,474,619 appears later in this discussion.
The present invention improves upon the prior art by alkoxylating the alkanolamine. This alkoxylation incorporates ether linkages into the conditioner which distinguish it structurally from coal conditioners of the prior art while providing unexpected improvements in clean coal recovery. For example, this process might employ a 1:3 reaction of DEA with propylene oxide, designated DEA-3PO. That product will then be esterified with TOFA, so one could speak of DEA-3PO-3TOFA. In order to facilitate discussion of the present invention in greater detail, the experimental particulars are set out below. The term "conditioners" as illustrated by the examples below is an agent added in a froth flotation process to enhance coal recovery selectivity and efficiency.